1. Technical Field of the Invention
The present invention relates generally to photolithography, and more particularly, to dynamic photolithography systems.
2. Description of Related Art
Photolithography is a method of transferring a pattern or image onto a substrate. Some industrial uses of photolithography include the manufacture of products, such as flat panel displays, integrated circuits (ICs), IC packaging, planar lightwave circuits (photonics), printed circuit boards, flexible circuits/displays and wafer bumping. In its simplest form, a photolithography system operates by passing light through a mask or tool placed over a substrate having a photosensitive surface, such as a layer of photoresist. Typically, the mask is formed of a transparent material with a fixed opaque pattern inscribed on the surface. Due to the photosensitivity of the substrate surface, when placed in contact with the mask and exposed to light, the pattern inscribed on the mask is transferred onto the substrate surface.
Most photolithography systems attempt to tightly control the processing conditions because variations in temperature and humidity can alter the shape or size of various surfaces. When the size of features to be photolithographically transferred onto the substrate surface is large, minor distortions in the substrate surface or mask can be accommodated using various alignment techniques. However, as technology has progressed and the feature size has decreased to 0.5 μm and smaller, conventional alignment techniques have been unable to adequately compensate for dimensional changes in either the substrate or the mask itself. In addition, traditional direct contact photolithography systems do not allow for corrections in the image size without requiring a new mask to be created, which is a costly and time-consuming process. Furthermore, even more advanced non-contact photolithography systems that use projection optics to separate the mask from the substrate are not able to correct for all types of distortions. For example, although simple global adjustments can be made optically using extra lenses, traditional non-contact photolithography systems cannot compensate for local distortions in the surface or distortions in the optical system itself.
Recently, dynamic photolithography systems have been developed that enable a pattern to be transferred onto a substrate surface without the use of a physical mask. Dynamic photolithography systems commonly employ a spatial light modulator (SLM) to define a pattern that is imaged onto the substrate surface. SLMs are electrically controlled devices that include individually controllable light modulation elements that define pixels of an image in response to electrical signals. Typically, at feature sizes of 0.5 μm or smaller, there are tens of millions of light modulation elements within an SLM, with each light modulation element being under 4 μm square.
However, each light modulation element is at a fixed location on the SLM, and each light modulation element can only be in one of two pixel states: full on or full off. As such, to resize the image transferred onto the surface to compensate for optical and surface distortions, the pixel data loaded into the SLM must be modified. For example, to stretch or shrink an image, an entire row or column of pixels in the image can be duplicated or deleted and rendered onto the array of light modulation elements. Unfortunately, the average error produced by duplicating or deleting pixels is ¼ pixel, and the peak error is ½ pixel. In addition, line widths can change up to ±50%, and diagonal lines can suffer from “jaggies” at each duplicate/delete point. An alternative to duplicating/deleting is to recompute the pattern and render the recomputed pattern onto the light modulation element array in real time to form a resized image. However, the many factors involved in computing the image, including optical system bandwidth, photoresist nonlinearities and adjacent features, make real time calculation difficult and expensive. In addition, recomputing the pattern/image for each image transfer is impracticable due to the high data rates (equivalent to 2,000 lap top screens of data/second) required in dynamic photolithography systems. Therefore, what is needed is a dynamic photolithography system capable of resizing an image in real time.